The SIR provided D,L-[15 N2-imidazoyl]Histidine; 1.48g Cleland and Kreevoy recently advanced the idea that a special type of hydrogen bond (H-bond), termed a low-barrier hydrogen bond (LBHB), may account for the "missing" transition state stabilization underlying the catalytic power of many enzymes, and Frey et al. have proposed that the H-bond between aspartic acid 102 and histidine 57 in the catalytic triad of serine proteases is an example of a catalytically important LBHB. Using "N labeled histidine provided by the SIR and NMR spectroscopy, we have shown that the aspartic acid-histidine and cis urocanic H-bonds that are inconsistent with fundamental tenets of the LBHB hypothesis. The inconsistencies between theory and experiment in these paradigm systems cast doubt on the existence of , as currently defined, within enzyme active sites.